In PSP you can flood fill with a pattern. Theres two boxes in the color tool bar one foreground and one background color. On each is a little arrow bottom right which opens up color type. They are No Color, Pattern, Solid and gradient. So choose pattern and pick one from the pattern swatch which includes all windowed images as patterns. So get some stone as an image on the screen and set it as the fill pattern and flood the walls with a nice stone.